In external beam radiation therapy, ionizing radiation is applied to target structures, such as tumors, within patients' bodies in order to control growth of or kill cancer cells. The radiation treatment is usually delivered in plural sessions, which are also referred to as treatment fractions in the art. In more advanced types of radiation therapy, such as so called intensity-modulated radiation therapy (IMRT), precise doses of radiation are applied to regions of the patient's body. In this respect, it is typically the goal to deliver a sufficiently high radiation dose to the target structure and to spare sensitive structures, such as organs, in the vicinity of the target structure as far as possible.
The treatment parameters for delivering the radiation and controlling the radiation treatment device are defined in a treatment plan. The treatment plan is generated in a planning system, which may particularly use a so-called inverse planning procedure. In such a procedure, the target structure and the surrounding structures to be spared are identified and treatment goals are specified. Such treatment goals include objectives which may specify requirements for the radiation dose delivered to certain regions of the patient, and/or constraints for the radiation doses delivered to certain regions. Then, an optimization process is carried out to find the treatment plan which fulfills the specified treatment goals. This optimization process is usually an operator-guided procedure in which an operator (e.g. a physician) reviews the dose distribution resulting from the treatment plan in several steps and makes changes to the treatment goals in order to find the optimal dose distribution.
Conventionally, such an inverse planning procedure is carried out on the basis of a stationary anatomical configuration of the region of interest, which does not change during the radiation treatment. However, the anatomical configuration of the region of interest does usually change during the radiation treatment. So, the delineation of a tumor changes due to its natural progression, which normally results in a growth of the tumor, and, most notably, due to the effects of the radiation therapy, which result in a (net) shrinkage of the tumor. Moreover, the position of the tumor can change during the time period between the delivery of two treatment fractions (so called inter-fraction motion) and relevant motion of the tumor can also occur during a single treatment fraction (so-called intra-fraction motion). The magnitude of such intra-fraction motion varies with the duration of the treatment fractions and usually also dependents on the body region comprising the tumor. One body region where larger magnitudes of intra-fraction motion of a tumor can occur is the prostate.
If the original treatment plan generated on the basis of the (stationary) anatomical configuration is used after shrinkage of the target structure and/or tumor motion, there is a high risk to affect healthy tissue by applying a high radiation dose to such tissue.
One approach for addressing this problem is the so-called adaptive radiation therapy. In accordance with this approach images of the region of interest are captured during the course of the radiation therapy in order to determine the changed anatomical configuration. Then, a re-planning procedure is carried out to adapt the treatment plan to the changed anatomical configuration. However, the re-planning procedure usually involves a very high computational complexity. In particular, it comprises a determination of the so-called influence matrix for the captured image. For each volume element (voxel) of the region of interest, the influence matrix quantifies the amount of dose absorbed by this voxel per unit intensity emission from all parts of the radiation beam (so-called beamlets). Due to the computational complexity involved particularly in the calculation of this matrix, the re-planning procedure is usually very slow.
Therefore, the re-planning in adaption radiation therapy has to be made “offline”, i.e. between the treatment fractions. As consequence, it is only possible to account for inter-fractional changes of the anatomical configuration of the region of interest, such as changes of the delineation of the target structure between two treatment fractions and inter-fraction motion of the target structure. However, it is not possible (without undue effort) to take account of intra-fractional changes of the anatomical configuration of the region of interest.
EP 1 977 788 A2 discloses a system for delivering radiation therapy to a moving region of interest. The system generates a set of 4D treatment plans for different breathing tracks of the region of interest, where each plan is optimized for a one breathing track. During the treatment delivery, the appropriate plan is particularly selected on the basis of a determined position of the region of interest, where the system particularly determines a track of the region of interest on the basis of the position data and selects the plan corresponding the determined track.